Biochemistry Lab Techniques for the MCAT Exam

Common failure modes

These are the patterns that cause most candidates to lose marks on this topic. Recognising them in advance is half the work.

• !Confusing native PAGE (preserves protein charge/shape) with SDS-PAGE (denatures proteins, separates only by size)
• !Not knowing that PCR requires a primer pair, denaturation, annealing, and extension steps — missing any step leads to wrong conclusions about experimental results
• !Confusing direct ELISA (detects antigen) with indirect ELISA (detects antibody) — the detection logic is inverted
• !Not relating centrifugation speeds to which cellular fraction pellets (nuclei pellet at low speed; mitochondria at medium; ribosomes/microsomes at high)

Study tips

• 1Draw the SDS-PAGE gel setup from memory: sample wells, stacking vs. resolving gel, direction of migration, and how to read molecular weight from a ladder.
• 2Write out the three PCR steps (denaturation at ~95°C, annealing at ~55°C, extension at ~72°C) and identify which enzyme does the extension and why Taq polymerase is used.
• 3Memorize the differential centrifugation sequence for cell fractionation: 600 × g (nuclei) → 10,000 × g (mitochondria/lysosomes) → 100,000 × g (ribosomes/ER fragments).
• 4Understand the sandwich ELISA workflow — primary antibody captures antigen, secondary antibody is enzyme-conjugated, substrate produces colorimetric signal — so you can answer questions about experimental controls.

Sample MCAT Biochemistry Lab Techniques questions

These sample items mirror the format and difficulty of real MCAT questions. Practice with thousands more on the free Koydo question bank.

1. 1

   A researcher runs an SDS-PAGE gel and observes that a recombinant protein migrates more slowly than expected based on its calculated molecular weight. The most likely explanation is:

   • AThe protein is degraded in the sample buffer
   • BThe protein has extensive post-translational glycosylation that increases its apparent molecular weightCorrect
   • CThe gel percentage is too low to resolve small proteins
   • DThe protein is not fully denatured and retains tertiary structure
   Why this answer?

   SDS-PAGE separates proteins by apparent molecular weight after denaturation. Glycosylation adds carbohydrate mass that migrates with the protein, increasing its apparent molecular weight on the gel beyond what the amino acid sequence predicts. This is a common explanation for discrepancies between calculated and observed migration positions of recombinant or secreted proteins. (Illustrative.)

2. 2

   PCR is performed using a template that contains a target sequence flanked by two primer-binding sites 500 bp apart. After 30 cycles, what is the predominant product?

   • AVariable-length fragments ranging from 500 bp to the full template length
   • BA discrete 500 bp fragment corresponding to the sequence between the primersCorrect
   • CTwo fragments: one 500 bp and one full-length template copy
   • DNo amplification occurs because the primers are too far apart
   Why this answer?

   PCR exponentially amplifies the sequence defined by the two primer-binding sites. After ~20+ cycles, short amplicons (the defined fragment) vastly outnumber long molecules that extend beyond the primer region. The predominant product after 30 cycles is the 500 bp fragment between the primers. Primer distance of 500 bp is well within the range of standard PCR amplification.

3. 3

   In an ELISA designed to measure serum antibody levels against a viral antigen, the plate is coated with viral antigen, patient serum is added, and then an enzyme-conjugated anti-human IgG antibody is added. This is an example of:

   • ADirect ELISA
   • BCompetitive ELISA
   • CIndirect ELISACorrect
   • DSandwich ELISA
   Why this answer?

   Indirect ELISA detects the presence and quantity of antibodies (not antigens) in a sample. The antigen is immobilized on the plate, the patient's antibodies bind, and then a secondary anti-IgG enzyme conjugate detects those antibodies. Direct ELISA uses a single antibody that is itself enzyme-tagged. Sandwich ELISA uses two antibodies to capture and detect an antigen.